The catalytic preparation of hydrocarbons from synthesis gas is well known in the art and is commonly referred to as Fischer-Tropsch synthesis.
Catalysts suitable for use in a Fischer-Tropsch synthesis process typically contain a catalytically active metal of Group VIII of the Periodic Table of the Elements (Handbook of Chemistry and Physics, 68th edition, CRC Press, 1987-1988). In particular, iron, nickel, cobalt and ruthenium are well known catalytically active metals for such catalyst. Cobalt has been found to be most suitable for catalysing a process in which synthesis gas is converted into primarily paraffinic hydrocarbons containing 5 or more carbon atoms. In other words, the C.sub.5+ selectivity of the catalyst is high.
Much research effort has been directed to finding catalysts, especially cobalt based catalysts, and/or process conditions resulting in a higher C.sub.5+ selectivity than known catalysts at the same or higher activity.
Thus, European patent specification No. 398 420 describes that the C.sub.5+ selectivity of catalysts comprising cobalt and zirconium, titanium or chromium on a porous carrier, having a small external surface area, can be improved by contacting the catalyst with a synthesis gas having a low hydrogen to carbon monoxide ratio, typically, from 1.1 to 1.2.
European patent specification No. 178 008 discloses cobalt catalysts supported on a porous carrier, wherein most cobalt is concentrated in the rim of the catalyst particle.
European patent specification No. 167 215 discloses a cobalt/zirconia on silica catalyst for use in a fixed catalyst bed which catalyst satisfies a relation between the internal surface area and the external surface area.
European patent specification No. 168 894 discloses an optimal activation procedure to increase the C.sub.5+ selectivity of a cobalt-based Fischer-Tropsch catalyst.
European patent specification No. 363 537 describes an increase in activity of cobalt catalysts supported on titania, by adding up to 15% by weight of silica to the titania carrier.
European patent application publication No. 498 976 describes catalysts containing cobalt and rhenium supported on a titania carrier. It is claimed that these catalysts have a high volumetric productivity (activity).
European patent application publication No. 71 770 describes a process for the preparation of linear .alpha.-olefins from synthesis gas. Inter alia cobalt/manganese and cobalt/vanadium catalysts are claimed to be applicable in this process. The C.sub.5+ selectivity of a catalyst comprising cobalt and manganese in a ratio of 1:6, is only 50%.
Van der Riet et al. (1986) J. Chem. Soc., Chem. Commun., pages 798-799 describe selective formation of C3 hydrocarbons from carbon monoxide and hydrogen using cobalt-manganese oxide catalysts. The cobalt/manganese ratio is typically 1:1.
International PCT application WO 93/05000 describes a Fischer-Tropsch catalyst comprising cobalt and scandium. Optionally, the catalyst contains additional promoters like thoria and/or other materials such as magnesia and manganese.
"The Fischer-Tropsch and Related Synthesis" by H. H. Storch, N. Golumbic, and R. B. Anderson (John Wiley and Sons, New York, 1951), referred to in International PCT Application WO 93/05000 provides a review of early work on Fischer-Tropsch catalysts, including catalysts comprising cobalt and manganese and/or vanadium. On page 120 reference is made to experiments in which it was found that cobalt-vanadium oxide and cobalt-manganese oxide catalysts were inactive as Fischer-Tropsch catalysts. However, on page 198 reference is made to experiments in which it was found that a catalyst containing cobalt and manganese in a atomic ratio of 6.2:1 had a higher C.sub.5+ selectivity as compared to a catalyst containing cobalt and thoria, but at a significantly lower synthesis gas conversion.
Australian patent application No. 46119/85 describes a catalyst containing cobalt, silica and a base or alkaline material, typically an alkali or alkaline earth metal. Optionally additional promoters may be present chosen from salts of elements chosen from the group of aluminum, magnesium, zinc, copper, manganese, chromium, vanadium, germanium, boron, molybdenum, lanthanum, the Rare Earths and the like or combinations thereof and arsenic or antimony. It is claimed that these catalysts have a high selectivity towards lower boiling 1-alkenes.
Despite the research effort in this field there is still room for improvement. Accordingly, it would be desirable if catalysts or process conditions could be found which result in a still higher C.sub.5+ selectivity at the same or, preferably, higher activity than known catalysts or process conditions.